(1) Field of the Invention
The present invention relates to an acrylonitrile-butadiene-styrene resin (hereinafter referred to as "ABS resin") composition. More particularly, the present invention relates to a wet-platable ABS resin composition and a plated article thereof.
(2) Description of the Prior Art
Plating of a resin is ordinarily carried out for imparting a metallic decorative effect, an abrasion resistance and a weatherability to the resin. ABS resins are especially used in this field because excellent plated articles can be easily obtained (e.g. "A Study of the Etching Effect on the Metal-to-ABS Surface Adhesion in Electroless Plating", S. P. Poa, C. C. Wan & C. J. Wu, Metal Finishing, Aug. 13-16 (1977); "A Study of Factors Affecting Peel Adhesion of Electroplated Coating; on ABS and Polypropylene", J. K. Dennis and P. Tipping, Electroplating and Metal Finishing, Nov. 9-16 (1974): Japanese Pat. Kokai Koho 54-103456).
In plated resin articles, however, the plating layer is really peeled from the resin surface according to changes of the ambient temperature because the difference of the linear expansion coefficient between the metal phase and the resin phase is very great, and even in ABS resins, it has been desired to improve the adhesion strength of the plating layer. In the field of vehicles, plated resin articles are especially used in large quantities so as to attain a light weight. In this field, a high adhesion strength enough to cope with severe application conditions is required for plating layers.
As means for improving the adhesion strength of a plating layer in ABS resins, there is known a method of increasing the content of the rubber component, a method of increasing the diameter of rubber particles and a method of increasing the acrylonitrile content. However, according to these methods, no practically sufficient adhesion strength can be obtained. Japanese Patent Application Laid-Open Specification No. 103456/79 teaches a technique of adding an organic silicon compound to an ABS resin. However, the degree of improvement of the adhesion strength of a plating layer attained according to this technique is insufficient, and since the organic silicon compound is expensive, this technique is disadvantageous from the economical viewpoint.
A plated resin article obtained by plating of a light-transmitting ABS resin, especially a partially plates resin article, has an information display function utilizing as a signal a change of the intensity of transmitted light and/or the color of transmitted light, because an unplated portion (non-plated portion) has a light-transmitting property. For example, the shape of the non-plated portion is utilized as a symbol indicating a function state to be discriminated by this symbol. As specific embodiments, there can be mentioned applications to switches and push buttons of light electric appliances, and if a shape of a letter is given to the non-plated portion, the on-off state of a switch or push button indicated by this shape can be displayed.
Although a plated resin article obtained by plating of a light-transmitting ABS resin is industrially valuable, only a very few products of this type are practically utilized. This is because it is difficult to form a tight wet-plating layer on a light-transmitting ABS resin.
In the production of a light-transmitting ABS resin, it is generally necessary to match the refractive index of the dispersed rubber phase with that of the continuous resin phase, and for this purpose, methyl methacrylate is ordinarily added to monomers constituting the continuous resin phase. However, we have found that where a compound having an ester bond is contained in the continuous resin phase, the adhesion of the plating layer in the final plated ABS resin article is extremely poor. In view of this fact, in order to obtain a plated article of a light-transmitting ABS resin having a plating layer excellent in the adhesion, it is considered necessary to subject a light-transmitting ABS resin, which does not contain a compound having an ester bond, that is, methyl methacrylate, in the continuous resin phase, to wet plating.